The present invention relates to variable load controlled fluid pressure regulator valves for establishing vehicle braking levels in accordance with different load conditions, and particularly to such regulator valves which comprise a load control piston subject to the pressure of two independent air springs of the vehicle, so that in the event the pressure of one air spring is lost, due for example to a ruptured bellows or broken pipe, the stroke of the control piston is reduced by one half.
For safety purposes, it is common practice to utilize two separate air springs of the vehicles, by forming the load control piston with different pressure faces on which the pressure of the respective air springs acts in opposition to a governing spring. A first cam surface of the load control piston actuates the regulator relay valve through a diaphragm type piston, the effective area of which may be varied in accordance with the vehicle load condition, in order to modulate the vehicle braking accordingly. An actuating stem of the diaphragm piston rides on the inclined control surface of the first cam member, so as to increase or decrease the diaphragm piston area, as the air spring pressure on the load control piston changes with the vehicle load condition between "full" load and "empty" load. A second cam member of the load control piston is provided with a surface that is inclined in the opposite direction to the inclined surface of the first cam member so as to adjust the effective diaphragm piston area, in the event there is a loss of air in at least one air spring, due to a rupture of the air spring bellows, for example.
Variable load brake regulator valves of the foregoing type are designed for a given set of conditions, wherein the actuator stem of the relay valve sets the variable area diaphragm piston for a predetermined "empty" load brake condition, when the load control piston is positioned so that the low point of the cam profile formed between the inclined control surfaces of the first and second cam members are engageable with the relay valve actuator stem. In the event one air spring ruptures, the load control force on the control piston is reduced by one half, thus changing the normal load dependent position of the control piston. If such a malfunction occurs when the vehicle is "empty", for example, the movement of the control piston places the second cam member in engagement with the actuator stem, thus preventing the braking pressure from being reduced below the "empty" load braking pressure.
Where the vehicle operating requirements are such that the "empty" load setting of the relay valve does not occur at the low point of the cam profile, the governing spring is adjusted to axially shift the control piston to cause the actuator stem to ride part way up the inclined control surface of the first cam member until the desired load setting is achieved. This results in a dangerous situation, since it is now conceivable that in the event an air spring ruptures under certain load conditions, the resultant movement of the load control piston will allow the actuator stem to ride down the incline of the first cam member, without engaging the incline of the second cam member. It is possible, therefore, that the resultant position of the actuator stem on the inclined surface of the first cam member will be in a lower position than in the "empty" load setting and will accordingly produce a lower braking force than the "empty" load braking force.